1. Field of the Invention
The invention relates to a solid-state imaging device, a method of manufacturing the solid-state imaging device, and an electronic apparatus, and more particularly to, a solid-state imaging device capable of more effectively suppressing noise, a method of manufacturing the solid-state imaging device, and an electronic apparatus.
2. Description of the Related Art
Recently, in a video camera or an electronic still camera, a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) image sensor is used as a solid-state imaging device for capturing an image. Particularly, in comparison with the CCD, the CMOS image sensor has excellent characteristics in terms of low power consumption.
In the related art, each pixel included in the CMOS image sensor is configured to include, for example, a photodiode, a transfer gate, a floating diffusion, a reset transistor, an amplification transistor, and a selection transistor. In addition, in each pixel, a read operation of reading out the signal charges, which are generated by the photodiode as the photoelectric conversion unit to be accumulated and transferred to the floating diffusion, through the amplification transistor is performed.
However, in the CMOS image sensor, since the read operation is performed for each row of the pixel array, the accumulation periods of the signal charges in all the pixels are not coincident with each other, and in the case were a subject is moved, distortion occurs in a captured image. For example, in the case of photographing an object, which is moving in the horizontal direction, straight in the up/down directions, the object is projected to be slanted.
In order to prevent occurrence of distortion in an image, an all-pixels simultaneous electronic shutter for the CMOS image sensor, in the exposure periods of the pixels are coincident with each other, has been developed. The all-pixels simultaneous electronic shutter is a shutter performing operations of simultaneously starting exposure of all the pixels effective for imaging and simultaneously ending exposure, which is also referred to as a global shutter (global exposure).
As a method of implementing the global shutter in the CMOS image sensor, there is, for example, a method of providing a memory portion between a photodiode and a floating diffusion for each pixel. In the CMOS image sensor having the pixels including the memory portion, for example, after exposure is simultaneously performed on the all pixels, the signal charges generated in the photodiodes are simultaneously transferred to the memory portions of the all pixels and temporarily accumulated in the memory portions. Next, the signal charges accumulated in the memory portions are transferred to the floating diffusions at a predetermined read timing to be converted into a voltage as an output.
However, for example, in the case where light is incident from an opening, through which light incident to the photodiode, toward an lower portion of the memory portion in a slanted direction, a portion of the charges generated in a relatively deep area under the memory portion through the photoelectric conversion may not be introduced into the photodiode but flowed into the memory portion. The charges flowed into the memory portion in this manner are read out similarly to the signal charges transferred from the photodiode. In this manner, the charges which are not transferred from the photodiode function as noise. Hereinafter, the output generated when the charges generated in the relatively deep area under the memory portion through the photoelectric conversion are flowed into the memory portion is referred to as noise.
In addition, generally, due to the existence of the P type well located in the deepest portion among the P type wells formed through multiple-stage injecting, a weak electric field area is formed between the junction position of the memory portion and the P type well located in the deepest portion. Therefore, the electrons generated in the weak electric field area through the photoelectric conversion are diffused toward the side of the memory portion with high probability, so that the noise is increased.
In order to suppress an increase in noise, a solid-state imaging device having a pixel structure where a P type layer is disposed through a portion of a P type well under an N type memory portion (charge storage unit) so that an impurity concentration of the P type layer is set to be higher than an impurity concentration of the P type well has been disclosed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2008-4692).